Síntese one-pot por combustão assistida por micro-ondas de catalisadores a base de Ni, Co e Fe para produção de H2 via reforma a seco do CH4

Abstract

Catalysts based on Ni, Co and Fe supported in Al2O3 were prepared simply and quickly by the one-pot microwave-assisted combustion method, using nitrates as precursors and low fuel content (urea). The catalysts were calcined at 550 ° C for 3 h and characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), N2 adsorption/desorption (BET-BJH), energy dispersive X-ray (EDX), scanning electron microscopy (SEM-EDX), transmission electron microscopy (TEM) and thermogravimetry (TG) with coupled Fourier Transform Infrared Spectrophotometry (FTIR). The structure of the catalysts during reducibility under H2 atmosphere and in the dry reforming reaction were investigated by in-situ X-ray diffraction. The catalytic tests were performed at a temperature of 700 ºC (activation and reaction) using a space velocity of 72 Lh-1 g -1 and a CH4/CO2 ratio of 1 for 20 h. The design of experiment was applied to study different formulations of the catalysts and to verify their effects on H2 yield. The results indicate that the active phases have crystallite sizes below 20 nm, high degree of reduction and specific surface area around 200 m²g -1 . In the catalytic tests different behaviors were observed, being the bimetallic Ni-Fe catalysts more stable and active than the bimetallic Ni-Co catalysts. The mathematical model for H2 yield with a correlation coefficient (R²) of 0.9977 complemented the results of the catalytic tests and was used to formulate the TA1 trimetallic catalyst (8.7 %wt. Ni, 1.3%wt. Co and 1.3%wt. Fe). Finally, in the optimization stage, the TA1 catalyst was promoted with Ce and Mg and activated at 800 ºC, aiming to reduce the coke formation that caused the encapsulation of the active phase, observed in all the catalysts tested. The activity of the oxygen vacancies and the modification of the surface basicity by CeO2 and MgO, respectively, strongly influenced the performance of the TA1 catalyst, in which deactivation levels were observed below 12 %, while the catalysts without the promoters presented deactivations above 40 %. Therefore, the promoters proved to be efficient in trimetallic catalysts containing nonnoble metals. The materials prepared in this thesis have potential for applications in heterogeneous catalysis, since the proposed route is simple, fast and uses low fuel, producing nanoscale catalysts with high resistance to coke formation, high stability and high yield of H2, in addition to allowing the partial replacement of Ni by components of lower cost and non-toxic as Fe.